1. Field of the Invention
The present invention relates to crystal-oriented ceramics which comprise an oxide having an isotropic perovskite-type structure, and to methods for producing them.
The terminology, isotropic perovskite-type structure as referred to herein means a crystal structure which is generally referred to as a perovskite-type structure but specifically has a cubic system or a slightly-distorted cubic (pseudo-cubic) system. In that meaning, the terminology, isotropic perovskite-type structure substantially contains both isotropic and pseudoisotropic perovskite structure. This isotropic perovskite-type structure herein referred to shall be definitely differentiated from a layered perovskite-type structure.
2. Description of the Related Art
Some proposals have heretofore been made for the technique of orienting the crystal planes or the crystal axes of polycrystalline ceramics. By orienting specific crystal planes or axes of polycrystalline ceramics, their characteristics that depend on the specific crystal planes or axes can be significantly improved. Through such orientation, therefore, it is possible to obtain polycrystalline ceramics having characteristics similar to those of single crystals.
Especially for ferroelectric polycrystalline ceramics, of which the characteristics greatly depend on their crystal axes having polarity, if the crystals constituting them are oriented, their characteristics based on the polarity, such as the amount of remnant polarization, are said to be improved over those of non-oriented polycrystalline ceramics in which the crystals constituting them are not oriented. Various patent applications and technical reports for such crystal-oriented polycrystalline ceramics have heretofore been made.
For magnetic materials, it is reported that magnetic heads comprising crystal-oriented ferrite ceramics have improved in abrasion resistance and therefore their life times are prolonged (see Powders and Powder Metallurgy, Vol. 26, No. 4, pp. 123-130, 1979).
Various means and methods of orienting polycrystalline ceramics have heretofore been disclosed, some of which are referred to hereinunder.
For example, when a polycrystalline ceramic having a layered perovskite-type structure such as typically bismuth titanate (Bi.sub.4 Ti.sub.3 O.sub.12), of which the surface energy at a specific crystal plane is much smaller than that at the other crystal planes, is hot-forged under uniaxial pressure with heating, it is converted into a dense, crystal-oriented ceramic in which the crystals constituting it are uniaxially oriented (see Jpn. J. Appl. Phys., Vol. 19, No. 1, pp. 31-39, 1980). This technique is to orient a substance having high crystal anisotropy in a stress field.
Substances having high crystal anisotropy, such as bismuth titanate mentioned above, can be produced in the form of powders of plate-like or needle-like grains. A method is known in which the powders having such morphological anisotropy are tape-cast in strips or extruded along with binders or liquids whereby they are oriented, and thereafter the shaped articles are sintered through heat treatment to obtain crystal-oriented ceramic articles (see J. Am. Ceram. Soc., Vol. 72, No. 2, pp. 289-293, 1989).
In the preprint of ISAF '96, page 211 (1996), blade-like grains of Sr.sub.2 Nb.sub.2 O.sub.7 and fine grains of Sr.sub.2 Nb.sub.2 O.sub.7 were mixed in such a ratio that the blade-like grains accounted for from 5 to 15% by volume, and sintered while orienting the blade-like grains to obtain sintered, crystal-oriented articles of Sr.sub.2 Nb.sub.2 O.sub.7.
In the preprint of ISAF '96, page 223 (1996), plate-like grains of Bi.sub.4 Ti.sub.3 O.sub.12 and fine grains of Bi.sub.4 Ti.sub.3 O.sub.12 were mixed in such a ratio that the plate-like grains accounted for from 5 to 15% by volume, and tape-cast into a sheet while orienting the plate-like grains, and the resulting strips cut out of the sheet, are laminated and sintered at from 900 to 1000.degree. C. to obtain sintered, crystal-oriented laminates of Bi.sub.4 Ti.sub.3 O.sub.12.
The technique common to these two reports are herein referred as TGG (templated grain growth).
In J. Am. Ceram. Soc., 78 [6], 1687-1690 (1995), Hirao et al. disclosed a method for obtaining sintered, crystal-oriented laminates, which comprises mixing .beta.-Si.sub.3 N.sub.4 grains (seeds), which are rod-like single crystals, and fine grains of .alpha.-Si.sub.3 N.sub.4 along with sintering aids, tape-casting the resulting mixture into a sheet through a doctor-blading device, and laminating the resulting strips cut out of the sheet.
The above-mentioned techniques are all to orient materials having morphological anisotropy in a stress field.
MO.6Fe.sub.2 O.sub.3 (where M denotes an element such as Ba, Sr, Pb or the like) having a magnetoplumbite-type structure is known as a typical hard ferrite, and is produced in various methods, such as solid phase or liquid phase methods, etc. (see J. J. Went, et al., Philips Tech. Rev. 13, 194 (1952); H. Yamamoto & R. Takeuchi, Powders and Powder Metallurgy, 43 (8), 984-989 (1996); Japanese Patent Application Laid-Open No. 56-50200).
This material has an axis of easy magnetization in its c-axial direction, and therefore the orientation of the material can be controlled by shaping or sintering the material in magnetic field. Through such controlled orientation, this material can be formed into a sintered, crystal-oriented body, of which the degree of c-axis orientation is, when measured according to X-ray diffractometry (in Lotgering method), more than 90% (see H. Taguchi, Electroceramics, July, 49-55 (1991); T. Shimoda, Electroceramics, July, 16-22 (1991)). In this case, since the large grains being oriented in the sintering step grow, while absorbing fine grains that are poorly orientable, it is known that the degree of orientation of the material increases with the growth of the large grains.
The crystal grains of such a material having a magnetoplumbite-type structure grow easily in the a-axial direction. Therefore, when the material is produced in a liquid phase method or the like, it is easy to obtain hexagonal plate-like grains of the material with morphological anisotropy having an expanded c-plane. In addition, it is possible to orient the grains of the material through compression molding, doctor blading or extrusion, in which are obtained sintered, crystal-oriented bodies with c-axial orientation (see Japanese Patent Application Laid-Open No. 55-154110). This technique is to orient the material with magnetic anisotropy and morphological anisotropy in a magnetic or stress field.
The above-mentioned techniques are to obtain oriented sintered bulks through the mechanism that comprises previously orienting a material with morphological anisotropy or magnetic anisotropy in a stress or magnetic field, followed by heating to thereby grow the oriented grains under heat. In this mechanism, the fine and poorly-orientable grains that exist along with the easily-orientable grains are not oriented and are absorbed by the oriented grains, while the oriented grains grow under heat.
However, these techniques are to attain the homoepitaxial growth of grains in a solid phase, and are therefore problematic in that they are applicable to the orientation of only materials with morphological or magnetic anisotropy to give crystal-oriented bulks of the materials.
It was possible to produce crystal-oriented ceramics such as spinel-type-structured ferrite according to so-called topotaxy, in which a powder mixture containing periodic-bond-chain-forming plate-like grains of, for example, .alpha.-type iron oxide, is shaped with increasing the degree of crystal orientation of the grains constituting the shaped product, whereupon the grains, and after the reaction upon heating, the reaction product, for example, ferrite inherit their orientation axes from the starting grains (see Electronic Ceramics, '91, July, pp. 56-63, 1991).
This technique is problematic in that it is effective only in the combination of the starting material that satisfies the steric lattice conformity applicable to topotaxy and the product from the starting material, for example, the combination of the starting material, iron oxide and the ceramic product, ferrite.
Apart from this, however, it was difficult to produce crystal-oriented ceramics having isotropic crystal forms of a cubic system or having pseudo-isotropic crystal forms of a slightly-distorted cubic system, if not starting from anisotropic materials with three-dimensional lattice conformity applicable to topotaxy. To produce such crystal-oriented ceramics, therefore, an expensive technique of growing single crystals was inevitable, and the producibility in this technique was poor.
Many ferroelectrics that are important in various engineering fields, such as typically PZT (compound name: lead zirconium titanate) and barium titanate, have a crystal form of a perovskite-type structure, which is a cubic structure or a slightly-distorted cubic structure, and their anisotropic characteristics greatly depend on the distorted direction.
However, the crystallographic anisotropy of these substances is very small, and it is therefore extremely difficult to produce powders with morphological anisotropy from these. In addition, the periodic-bond-chain-forming oxides of Ti, Zr, Nb or the like in these are similar to the periodic-bond-chain-forming oxides in perovskite-type-structured substances in terms of the long-range structure unit, and powders of these substances with morphological anisotropy cannot be produced. Therefore, it was difficult to control the orientation of these substances through topotaxy (see K. Kugimiya & K. Hirota, Electroceramics, July, pp. 56-63 (1991)).
Patent publications were issued which relate to a technique of producing oriented ceramics of lead titanate or barium titanate from potassium titanate fibers or their derivatives, fibrous titanium oxide and fibrous titanium oxide hydrate (see Japanese Patent Publication Nos. 63-24949, 63-24950, 63-43339, 63-43340, 63-43341). In principle, however, it is extremely difficult to produce crystal-oriented ceramics from potassium titanate fibers and their derivatives having a Ti--O bond network that is different from the network of a perovskite-type structure.
This is because, even if grains of potassium titanate fibers and their derivatives could be oriented, the reaction to produce perovskite-type-structured compounds from them shall inevitably involve the re-arrangement of the Ti--O bond skeleton, and it is extremely difficult to still maintain the crystal orientation during the re-arrangement.
Another method of obtaining crystal-oriented ceramics having a crystal-oriented, perovskite-type structure comprises forming a thin film on a substrate through sputtering, chemical vapor deposition (CVD), sol-gel deposition or the like. To this, applicable is a known technique of epitaxy that shall occur between the specific crystal plane in the perovskite-type structure and the surface of the substrate having good lattice conformity with the specific crystal plane, or of self-texture that brings about the orientation of the specific crystal plane irrespective of the crystal orientation of the substrate but owing to the difference in the surface energy or the difference in the supply of elements.
However, this method is problematic in that it takes much time to produce thicker films, resulting in the increase in the production costs. In this method, in addition, the film to be formed is restrained by the substrate. Therefore, if a thicker film is formed in this method, it is often cracked or peeled from the substrate during heat treatment, due to the stress resulting from the crystallization and densification of the film or due to the difference in the degree of thermal expansion between the film and the substrate. If so, the film formed is broken.
For these reasons, it is extremely difficult to obtain crystal-oriented ceramic films having a thickness of larger than 5 .mu.m, according to this method.
Therefore, this method is unsuitable for the production of bulky materials. As has been mentioned hereinabove, it was difficult to produce crystal-oriented ceramics having a perovskite-type structure, except for the expensive technique of growing single crystals with poor producibility.
It is reported, for example, perovskite-type single crystals with a rhombohedral morphotropic phase exhibited excellent piezoelectric properties when poled in the &lt;111&gt; direction as expressed in the form of a pseudo-cubic system (see Journal of Applied Physics, Vol. 82 No. 4, pp. 1804-11, 1997). However, the production of such single crystals were expensive technique and the small sizes of the obtained single crystals limited their applications.
The alignment of crystal axes could also enhance the piezoelectric properties of the lead-free and thus environmentally benign piezoelectric ceramics in the perovskite-type structure. Bi.sub.0.5 Na.sub.0.5 TiO.sub.3 and its solid solutions are known as promising candidates for lead-free or -less-containing piezoelectrics since they have relatively high electromechanical coupling factor and good sinterability, as reported in several documents (Japanese Examined Patent Publication No. 4-60073; Silicates Industries, No. 7/8, 136-142, 1993; Sensors and Materials, Vol. 9, No. 1, 47-55, 1997). However, the Bi.sub.0.5 Na.sub.0.5 TiO.sub.3 and its solid solutions without crystal orientation gives relatively smaller planar electromechanical coupling coefficients (Kp) and transverse piezoelectric properties (d.sub.31 and g.sub.31) Known are crystal-oriented ceramics, in which a plurality of crystal planes or axes are oriented in three-dimensional orientation.
Crystal-oriented ceramics of that type can be produced by hot-pressing ceramic grains with morphological anisotropy in different directions. Japanese Patent Publication No. 01-32186 discloses the production of sintered bodies with three-dimensional orientation directly from grains with morphological anisotropy.
Japanese patent applications were laid open, which relate to the production of spinel-type-structured magnetic ceramics with three-dimensional orientation by extruding flaky or needle-like grains and in which the grains are three-dimensionally oriented through topotaxy (see Japanese Patent Application Laid-Open Nos. 49-129892, 56-21810, 56-27902).
However, according to the above-mentioned techniques, it was impossible to produce crystal-oriented ceramic bulks having isotropic crystal forms of a cubic system or having pseudo-isotropic crystal forms of a slightly-distorted cubic system, if not starting from anisotropic materials with three-dimensional lattice conformity applicable to topotaxy. To produce such crystal-oriented ceramic bulks, an expensive technique of growing single crystals was inevitable, and the producibility in this technique was poor.
For producing devices with excellent characteristics by orienting the crystal axis of a functional thin film comprising an isotropic perovskite-type compound, such as PZT (zirconium lead titanate), or a functional thin film comprising a layered perovskite-type compound such as a high-temperature superconductor, it is known to form the functional thin film on a single-crystalline substrate, for example, MgO, Al.sub.3 O.sub.3, or an isotropic perovskite-type compound, such as SrTiO.sub.3, which has epitaxial relation to the compound of the film being formed.
In particular, it is especially preferred to use a substrate comprising single crystals of SrTiO.sub.3 or the like isotropic perovskite-type compound having good lattice conformity to the substance of the thin film to be formed on the substrate. In this preferred case, obtained are thin films with better crystallinity and orientation.
According to the above-mentioned method, it is possible to form a functional thin film comprising the above-mentioned compound, directly on the above-mentioned single-crystalline substrate. If desired, electrodes or other devices may be formed on the single-crystalline substrate, and a functional thin film comprising the above-mentioned compound may be formed thereover.
Where electrodes are formed on the single-crystalline substrate, an electroconductive thin film of a metal such as Pt, or of an electroconductive perovskite-type compound such as LaNiO.sub.3, which has epitaxial relation to the substrate, is formed on the substrate, and thereafter a functional thin film comprising the above-mentioned compound is formed over the electroconductive thin film through sol-gel deposition, sputtering, laser ablation or the like. In this process, the functional thin film formed is epitaxially oriented relative to the single-crystalline substrate.
One example is disclosed in J. Appl. Phys., 60 [1], 361-367 (1986), in which is formed a thin oriented film of PbTiO.sub.3 on a substrate of epi{100}Pt/{100}MgO single crystals.
Jpn. J. Appl. Phys., 16, 1707-1708 (1977) discloses the formation of a thin, epitaxially-oriented film of (Pb,La)(Zr,Ti)O.sub.3 on a substrate comprising single crystals of SrTiO.sub.3.
Japanese Patent Application Laid-Open Nos. 6-310769 and 7-309700 disclose the formation of a thin, high-temperature superconductive film comprising a layered perovskite-type compound on a substrate comprising single crystals of SrTiO.sub.3. In addition, it is known to form a thin ferroelectric film having an isotropic perovskite-type structure over the high-temperature superconductive thin film, while using the high-temperature superconductive thin film as an electrode that acts also as a template.
Appl. Phys. Lett., 69 [22], 3432-3434 (1996) discloses a device as produced by forming a thin film of isotropic perovskite-type-structured Nd.sub.0.7 Sr.sub.0.3 MnO.sub.3 having a giant magneto-resistivity effect on a substrate of single crystals of LaAlO.sub.3, followed by further forming a thin film of isotropic perovskite-type-structured YBa.sub.2 Cu.sub.3 O.sub.7 thereover.
As the single-crystalline substrate, mostly used are isotropic perovskite-type-structured substrates. This is because isotropic perovskite-type-structured or layered perovskite-type-structured oxide-type single-crystalline substrates are usable as templates for uniaxial orientation and epitaxial growth of most thin, isotropic perovskite-type-structured, functional films.
However, isotropic perovskite-type-structured oxide single-crystalline substrates are expensive, and large-area wafers are difficult to form thereon. Therefore, even if various thin films are formed on such expensive single-crystalline substrates, it is difficult to obtain low-priced devices. In addition, if large-area devices (having an area of 100 mm.sup.2 or larger) are formed on such single-crystalline substrates, the yield of the devices is low.
In addition, commercially-available, oxide-type single-crystalline substrates are limited. For example, for isotropic perovskite-type compounds, only SrTiO.sub.3, LaAlO.sub.3 and the like are commercially available. Given this situation, it is difficult to obtain single-crystalline substrates having good lattice conformity with functional thin films to be epitaxially grown on the substrates. If the lattice conformity of the single-crystalline substrate with the functional thin film formed thereon is poor, the degree of orientation of the functional thin film formed is low, resulting in that the property of the device formed is unfavorable.